WO2006111824A1

WO2006111824A1 - Method for controlling a humidity sensor, and corresponding humidity-sensing device

Info

Publication number: WO2006111824A1
Application number: PCT/IB2006/000918
Authority: WO
Inventors: Nadia Camaioni; Giuseppe Casalbore-Miceli; Alessandro Martelli
Original assignee: Consiglio Nazionale Delle Ricerche
Priority date: 2005-04-19
Filing date: 2006-04-18
Publication date: 2006-10-26
Also published as: EP1877763A1; ITBO20050250A1

Abstract

A humidity-sensing device (1) uses a humidity sensor (8) provided with a solid electrolytic material; the sensor (8) is supplied with a control voltage (VP) having an alternating waveform, which has: two voltage pulses (23, 24) having mutually opposite polarities, one and the same amplitude (VPm) , and one and the same first duration (TP) that is shorter than a maximum electrical-excitation time (Tmax) , below which the impedance of the sensor (8) depends only upon the oscillation of charges about a position of equilibrium, and is greater than or equal to a relaxation time (Tmin) of response to an electrical excitation of the sensor (8) ; and a period (25) of inactivity falling between the two pulses (23, 24) and having a second duration (TI) greater than or equal to the relaxation time (Tmin) .

Description

METHOD FOR CONTROLLING A HUMIDITY SENSOR, AND CORRESPONDING HUMIDITY-SENSING DEVICE

TECHNICAL FIELD The present invention relates to a method for controlling a humidity sensor and to a corresponding humidity-sensing device .

In particular, the present invention finds advantageous application in the case of a sensor having as humidity- sensitive element a film of solid electrolytic material having an impedance that varies as a function of humidity. The electrolytic material usually comprises an ionic constituent. Examples of solid electrolytic materials sensitive to humidity are:

• polymers containing ionic groups, usually referred to as "poly-electrolytes";

• polymer-based charge-transfer ("donor- acceptor") complexes and polymer-salt complexes; or else

• compounds of an ionic type obtained by doping oligomers or molecules with high molecular weight.

In particular, some examples of electrolytic materials are described in: the article "Polymer electrolytes and humidity sensors: progress in improving an impedance device", M.J. Yang et al.,

Sensors and Actuators B 86 (2002) pp. 229-234, Elsevier;

- the patent No. CN1384354A; and - the patent No. US6568265B1.

When an electrolytic material sensitive to humidity comes into contact with water, the ions of the ionic constituent become mobile so causing an increase in the electrical conductivity of the material, which electrical conductivity depends upon the characteristics of the ionic constituent (intermolecular structure, ion concentration, etc.) and upon the amount of water absorbed.

BACKGROUND ART The sensor normally comprises a pair of electrodes, which are set in contact with the film of electrolytic material and project outwards from the film itself. From the electrical standpoint, the film of electrolytic material can be represented schematically as an impedance constituted by a resistance and a capacitance set in parallel, the values of which depend, according to a non-linear relation, upon the percentage of relative humidity (RH%) in contact with the sensor, and depend also upon the temperature of the sensor.

In particular, when an excitation voltage is applied across the electrodes of the sensor, the sensor responds with an electric current circulating through the electrolytic material, which electric current depends upon the amplitude of the excitation voltage, upon the electrical conductivity of the electrolytic material itself and upon the capacitance of the electrolytic material and of the electrodic interphases, namely, the portions of film of the electrolytic material set in contact with the electrodes. Hence, in ultimate analysis, the electric current is found to be a function of the humidity in contact with the sensor. In particular, the amount of current increases as the amplitude of the excitation voltage increases .

When large amplitudes of the excitation voltage are used, there is an inevitable accumulation of charges of opposite sign on the electrodes, which accumulation destroy the symmetry of the sensor and vary the electrodic-interphase potential. Said variation in the potential can trigger redox reactions with consequent and irreversible modification of the sensor. In fact, said reactions develop highly reactive molecules, which can damage the electrolytic material. In order to prevent said drawback, it has been proposed to apply across the electrodes an alternating excitation voltage with zero mean value, for example, a square wave oscillating at a given frequency. Said approach has not proved a satisfactory solution, and irreversible chemico-physical modifications of the electrolytic material are still noted, in particular at the electrodic interphases.

In addition, is has also been noted that the circulation of current causes, after long times of use, an inevitable heating of the sensor by the Joule effect, said heating causing a change in the microclimate in the proximity of the sensor such as to falsify the measurement of humidity.

DISCLOSURE OF INVENTION

The aim of the present invention is to provide a method for controlling a humidity sensor and a humidity-sensing device which will be free from the drawbacks described above.

In accordance with the present invention a method for controlling a humidity sensor and a humidity-sensing device are provided according to the annexed claims .

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described with reference to the annexed plate of drawings, which illustrates a non- limiting example of embodiment thereof and in which:

• Figure 1 is a block diagram of a humidity-sensing device made in accordance with the present invention;

• Figure 2 illustrates a preferred embodiment of a humidity-sensing unit of the device of Figure 1;

• Figure 3 illustrates a waveform of a control voltage applied to the humidity-sensing unit of Figure 2; • Figure 4 illustrates a Cartesian diagram representing the variation in the impedance (in absolute value) of a humidity sensor of the humidity- sensing unit of Figure 2 as a function of the relative humidity (RH%) ; and

• Figure 5 illustrates an example of a Nyquist diagram of the frequency response of the humidity sensor of the humidity-sensing unit of Figure 2 for two different values of relative humidity (RH%) .

BEST MODE FOR CARRYING OUT THE INVENTION In Figure 1, the reference number 1 designates, as a whole, a humidity-sensing device comprising: a humidity-sensing unit 2 for detecting the humidity in the proximity of the humidity- sensing device 1 itself; an electrical-pulse generator 3 designed to apply an alternating control voltage VP to an input 4 of the humidity-sensing unit 2; a processing unit 5, which is designed to receive at input, from an output 6 of the humidity-sensing unit 2, a voltage VO of response to the control voltage VP, and to supply at output a measurement of humidity processed on the basis of the voltage VO; and an acquisition unit 7, designed to record and display the measurements of humidity supplied by the processing unit 5.

The humidity-sensing unit 2 in turn comprises a humidity sensor 8 and an amplification circuit 9, which has an output directly connected to the output β of the humidity-sensing unit 2 and is designed to receive at input and to amplify a current I circulating in the sensor 8 due to application of the control voltage VP. The sensor 8 is of the type comprising a film (not illustrated) made of solid electrolytic material sensitive to humidity having an impedance Z that varies as a function of the relative humidity (RH%) according to a known non-linear relation (illustrated in Figure 4) .

The processing unit 5 comprises a digital processor 10 of a known type, for example a personal computer, an analog-to- digital (A/D) converter 11 cascaded to a sampling device 12 (Sample/Hold) for converting the voltage signal VO into a digital form suitable for being treated by the digital processor 10, and a timing device 13 controlled by the digital processor 10 and designed to control the sampling device 12 and the converter 11. The timing device 13 is moreover connected to a control input 14 of the pulse generator 3, the digital processor 10 controlling generation of the control voltage VP through said input 14.

Finally, the digital processor 10 comprises a memory 15 designed to store a table containing a mapping between values of impedance Z of the sensor 8 and corresponding values of relative humidity (RH%) , said mapping being determined experimentally and being substantially similar to the diagram of Figure 4, and a further table of compensation of the temperature drifts of the sensor 8 according to a relation of compensation known and determined experimentally.

According to what is illustrated in Figure 2, the amplification circuit 9 comprises an operational amplifier 16 having an output, which is directly connected to the output 6 of the humidity-sensing unit 2, an inverting input 17, connected in series to which is the sensor 8, and a resistor 18 having a resistance Rl connected in feedback relation to the operational amplifier 16 between the output 6 and the inverting input 17 itself. In addition, a resistor 19 is connected between a non-inverting input 20 of the operational amplifier 16 and an electrical ground 21 of the humidity- sensing device 1 and has a value R2 sized so as to compensate for any possible offset currents of the operational amplifier 16. According to what is illustrated in Figure 2, the operational amplifier 16 is configured as a current-voltage converter designed to transform, via a gain of value Rl, the current I circulating in the sensor 8 into the voltage VO according to the linear relation:

VO = - Rl-I. (1) The value Rl of the resistor 18 is determined in such a way that the smallest value of current I produced by the sensor 8 can be detected, in terms of voltage VO, by the processing unit 5.

Once again with reference to Figure 2, the humidity-sensing unit 2 comprises a resistor 22 of a given value R3, which is set in series to the sensor 8, between the input 4 and the sensor 8 itself, and is designed to limit the electric power dissipated in the sensor 8 at high values of relative humidity

(RH%) and to cause the operational amplifier 16 to work far away from the saturation area.

According to what is illustrated in Figure 3, the control voltage VP has a waveform comprising: a succession of two voltage pulses 23 and 24, which have mutually opposite polarities, in particular the first pulse 23 has a negative polarity and the second pulse 24 has a positive polarity, have one and the same amplitude VPm, where the amplitude VPm is understood as the maximum absolute value of the two pulses 23 and 24, and have one and the same duration TP; and a period of inactivity 25 falling between the pulses 23 and 24 and having a duration TI preferably, but not necessarily, greater than the duration TP. In addition, the pair of pulses 23 and 24 set at an interval apart by the period of inactivity 25 is repeated at a frequency FP determined in such a way that the various repetitions of pairs of pulses 23 and 24 will be interspersed by a resting period 26 having a duration TR much greater than the durations TI and TP.

The value of the amplitude VPm is chosen so as to have a high sensitivity of the sensor, i.e., so as to have an appreciable current I for the values of relative humidity of interest.

The value of the duration TI is greater than or equal to a minimum time constant, for simplicity designated hereinafter as Train, which corresponds to the relaxation time of the current I circulating in the sensor 8 following upon application across the electrodes of the sensor 8 of a voltage step of amplitude VPm.

In this regard, it should be emphasized that a solid electrolytic material responds, following upon application of a voltage step, with the circulation of a current which as a whole has a short initial peak and a subsequent decay portion, also known as "electrical relaxation", which converges towards a value of equilibrium. In other words, during electrical relaxation, the current can be substantially represented by a superposition of two components of current: an equilibrium current constant in time; and a relaxation current having a time evolution that depends upon the characteristics of the electrolytic material and that follows, in the majority of cases, a law of the type: I(t) = A-t^"n. (2)

The relaxation time can be defined experimentally as the time after which, starting from the instant of application of the voltage step of amplitude VPm, the relaxation current assumes substantially negligible values, i.e., the total current is substantially equal to the equilibrium current.

Electrical relaxation in polymers is described, for example, in the article "Proprieta elettriche dei polimeri", by F. Sandrolini, in the volume "Macromolecole, Scienza e Tecnologia", F. Ciardelli et al. , vol. 2, p. 388, Pacini Editore (1982) .

The value of the duration TP is greater than or equal to the minimum time constant Tmin and, at the same time, sensibly shorter than a maximum time constant, designated hereinafter by Tmax, corresponding to the algebraic inverse of a minimum excitation frequency Fmin, which can be obtained through measurements of impedance spectrometry represented graphically by Nyquist diagrams.

Figure 5 shows an example of Nyquist diagrams which represent the frequency response of the sensor 8 for two different values of relative humidity, and in particular for an RH of 65% (full circles) and for an RH of 55% (empty circles) . The value of the minimum frequency Fmin, and hence of the respective time constant Tmax, is determined in a point corresponding to a minimum of the imaginary component Z" of the impedance Z of the Nyquist diagram. For frequencies higher than the minimum frequency Fmin, the impedance Z of the sensor 8 depends only upon the oscillation of charges about a position of equilibrium, whereas, for frequencies lower than the minimum frequency Fmin, Faraday phenomena affecting water, which can cause damage of the solid electrolytic material of the sensor 8, become predominant. As may be noted in Figure 5, the value of Tmax decreases as the relative humidity at which the Nyquist diagram is drawn increases; consequently, in order to define the duration TP, the value Tmax considered is the one corresponding to the highest value of relative humidity at which the humidity-sensing device 1 is designed to operate.

It should be noted that the definition of the durations TI and TP is subordinate to the choice of the type of sensor 8 and to the definition of the amplitude VPm. In other words, once the type of sensor 8 has been chosen, it is necessary first of all to fix the amplitude VPm that maximizes the sensitivity of the sensor 8 itself, after which Tmin is determined by observing the evolution of the relaxation of the current I that follows upon application across the sensor 8 of a pulse of amplitude

VPm, and finally Tmax is obtained from the Nyquist diagrams obtained by applying across the sensor 8 a square wave of amplitude VPm.

According to a preferred embodiment, the sensor 8 is of the type described in the patent No. CN1384354A (or else in the patent No. CN2543063A) , to which the reader is referred for further details, for said sensor 8 the optimal choice being to apply pulses 23 and 24 which have a amplitude VPm of 4 V, and a relaxation time, and hence a minimum time constant Tmin, of approximately 16 μs and a maximum time constant Tmax ranging from 100 μs to 200 μs, and hence approximately ten times greater than Tmin.

It is, moreover, advantageous to fix the duration TP at a value of between 10 μs and 24 μs, and preferably of 16 μs, and a duration TI at a value of between 35 μs and 50 μs, and preferably of 40 μs .

Finally, it is sufficient to repeat the pair of pulses 23 and 24 at a frequency FP of at least 0.5 Hz, and preferably of 4 Hz, corresponding to a duration TR at least four orders of amplitude greater than the durations TP and TI.

As regards, instead, sizing of the components of the humidity- sensing unit 2, the operational amplifier 16has bias currents of a value preferably smaller than 500 pA and a slew-rate factor of a value higher than 18 V/μs, the value Rl of the resistor 18 is 680 kΩ and the value R3 of the resistor 22 is between 50 and 100 kΩ.

In use, application of the negative pulse 23 of a value -VPm causes the sensor 8 to generate a current pulse I in the branch between the input 4 and the inverting input 17. The duration TP of the pulse (substantially equal to the relaxation time Tmin) is such as to enable stabilization of the current pulse I at an equilibrium value -Im depending upon the humidity.

The duration TI of the subsequent period of inactivity 25 (substantially greater than the relaxation time Tmin) is such as to enable redistribution of charges in the sensor 8 before arrival of the positive pulse 24 of value VPm. The positive pulse 24 is necessary to obtain a waveform with zero mean value that preserves the symmetry of the sensor 8, in so far as it stimulates the sensor 8 to produce a current pulse I of a sign opposite to the one generated by the negative pulse 23, namely, one having an equilibrium value Im.

Said negative and positive current pulses I of values -Im and Im, respectively, are amplified and transformed into respective positive and negative voltage pulses VO of values VOm and -VOm, respectively, according to Eq. (1) .

At the end of the positive pulse 24, there follows the resting period 26, the duration TR of which enables dissipation of the heat accumulated by the sensor 8 by the Joule effect during application of the pulses 23 and 24.

Claims

1.- A method for controlling a humidity sensor (8), which comprises a solid electrolytic material; the method envisaging supplying the sensor (8) with an alternating control voltage (VP) , which has a waveform comprising a first voltage pulse (23) and a second voltage pulse (24) that have mutually opposite polarities, one and the same amplitude (VPm) and one and the same first duration (TP) ; the method being characterized in that the first duration (TP) is shorter than a maximum electrical-excitation time (Tmax) , below which the impedance of the sensor (8) depends only upon the oscillation of charges about a position of equilibrium.

2.- The method according to Claim 1, in which said first duration (TP) is shorter than a value equal to 50% of said maximum electrical-excitation time (Tmax) .

3.- The method according to Claim 2, in which said first duration (TP) is shorter than a value equal to 30% of said maximum electrical-excitation time (Tmax) .

4.- The method according to any one of the preceding claims, in which said first duration (TP) is greater than or equal to a relaxation time (Tmin) of response to an electrical excitation of the sensor (8) .

5.- The method according to any one of the preceding claims, in which said waveform of the control voltage (VP) comprises a period (25) of inactivity, which falls between said first pulse (23) and said second pulse (24) and has a second duration (TI) greater than or equal to a relaxation time

(Tmin) of response to an electrical excitation of the sensor

(8) .

6.- The method according to Claim 5, in which said second duration (TI) is greater than a value equal to 50% of said relaxation time (Train) .

7.- The method according to any one of the preceding claims, in which the series constituted by said first pulse (23) and said second pulse (24) are repeated at a frequency (FP) greater than or equal to 0.5 Hz, and preferably of 4 Hz.

8.- The method according to any one of the preceding claims, in which said amplitude (VPm) of said first pulse (23) and said second pulse (24) has a value greater than 3 V, and preferably of 4 V.

9.- A humidity-sensing device, comprising: - humidity-sensing means (2), which have a control input (4) and an output (6) and comprise a humidity sensor (8), which, in turn, comprises a solid electrolytic material;

- electrical-pulse generation means (3) designed to supply to the control input (4) an alternating control voltage (VP), which has a waveform comprising a first voltage pulse (23) and a second voltage pulse (24) which have mutually opposite polarities, one and the same amplitude (VPm) and one and the same first duration (TP) ; and

- a processing unit (5) designed to receive from the output (6) of the humidity-sensing means (2) a voltage (VO) of response of the humidity-sensing means (2) to the control voltage (VP) , and to supply a measurement of humidity processed on the basis of the response voltage (VO) ; the humidity-sensing device (1) being characterized in that the first duration (TP) is shorter than a maximum electrical- excitation time (Tmax) below which the impedance of the sensor (8) depends only upon the oscillation of charges about a position of equilibrium.

10.- The device according to Claim 9, in which said first duration (TP) is shorter than a value equal to 50% of said maximum electrical-excitation time (Tmax) .

11.- The device according to Claim 10, in which said first duration (TP) is shorter than a value equal to 30% of said maximum electrical-excitation time (Tmax) .

12.- The device according to any one of Claims 9 to 11, in which said first duration (TP) is greater than or equal to a relaxation time (Tmin) of response to an electrical excitation of the sensor (8) .

13.- The device according to any one of Claims 9 to 12, in which said electrical-pulse generation means (3) are designed to insert, between said first pulse (23) and said second pulse (24) of the waveform of the control voltage (VP), a period

(25) of inactivity having a second duration (TI) greater than or equal to said relaxation time (Tmin) .

14.- The device according to Claim 13, in which said second duration (TI) is greater than a value equal to 50% of said relaxation time (Tmin) .

15.- The device according to any one of Claims 9 to 14, in which said electrical-pulse generation means (3) are designed to repeat the series constituted by said first pulse (23) and said second pulse (24) with a frequency (FP) greater than or equal to 0.5 Hz, and preferably of 4 Hz.

15.- The device according to any one of Claims 9 to 15, in which said control voltage (VP) causes a circulation of a current (I) in the sensor (8); said humidity-sensing means (2) comprising amplification means (9), which have an output directly connected to said output (6) of the humidity-sensing means (2) and are designed to perform a linear processing on the current (I) for supplying to the output (6) the voltage

(VO) of response to the control voltage (VP) .

17.- The device according to Claim 16, in which said amplification means (9) comprise an operational amplifier (16), which has an inverting input (17) and an output directly connected to said output (6) of said humidity-sensing means (2), and a first resistor (18) of a given value (Rl) connected in feedback relation between the output of the operational amplifier (16) and the inverting input (17) .

18.- The device according to any one of Claims 9 to 17, in which said humidity-sensing means (2) comprise a current- limiter element (22), which is connected in series to said sensor (8), between said control input (4) and the sensor (8) itself, and is designed to limit the power dissipated in the sensor (8) itself for high values of relative humidity.

19.- The device according to Claim 18, in which said current- limiter element (22) is constituted by a second resistor (22) of a given value (R3) .

20.- The device according to any one of Claims 9 to 19, in which said amplitude (VPm) of said first pulse (3) and said second pulse (24) has a value greater than 3 V, and preferably of 4 V.

21.- The device according to any one of Claims 17 to 20, in which said operational amplifier (16) has a bias current of a value smaller than 500 pA and a slew-rate factor greater than 18 V/μs; said first resistor (18) has a value (Rl) of 680 kΩ; and said second resistor (22) has a value (R3) of between 50 kΩ and 100 kΩ.

22.- A method for controlling a humidity sensor (8), which comprises a solid electrolytic material; the method envisaging supplying the sensor (8) with an alternating control voltage (VP) , which has a waveform comprising a first voltage pulse (23) and a second voltage pulse (24) that have mutually opposite polarities, one and the same amplitude (VPm) , and one and the same first duration (TP) ; the method being characterized in that the waveform of the control voltage (VP) comprises a period (25) of inactivity, which falls between said first pulse (23) and said second pulse (24) and has a second duration (TI) greater than or equal to a relaxation time (Tmin) of response to an electrical excitation of the sensor (8 ) .

23.- A method for controlling a humidity sensor (8), which comprises a solid electrolytic material; the method envisaging supplying the sensor (8) with an alternating control voltage (VP) , which has a waveform comprising a first voltage pulse (23) and a second voltage pulse (24) that have mutually opposite polarities, one and the same amplitude (VPm) , and one and the same first duration (TP) ; the method being characterized in that the first duration (TP) is shorter than a maximum electrical-excitation time (Tmax) below which the impedance of the sensor (8) depends only upon the oscillation of charges about a position of equilibrium; and the waveform of the control voltage (VP) comprises a period (25) of inactivity, which falls between said first pulse (23) and said second pulse (24) and has a second duration (TI) greater than or equal to a relaxation time (Tmin) of response to an electrical excitation of the sensor (8) .

24.- A method for controlling a humidity sensor (8), which comprises a solid electrolytic material; the method envisaging supplying the sensor (8) with an alternating control voltage (VP) , which has a waveform comprising a first voltage pulse (23) and a second voltage pulse (24) that have mutually opposite polarities, one and the same amplitude (VPm) and one and the same first duration (TP) ; the method being characterized in that the first duration (TP) is shorter than a maximum electrical-excitation time (Tmax) below which the impedance of the sensor (8) depends only upon the oscillation of charges about a position of equilibrium and is, moreover, greater than or equal to a relaxation time (Tmin) of response to an electrical excitation of the sensor (8); and the waveform of the control voltage (VP) comprises a period (25) of inactivity, which falls between said first pulse (23) and said second pulse (24) and has a second duration (TI) greater than or equal to said relaxation time (Tmin) .